A weather radar transmits pulses of very short duration followed by listening time. The position and intensity of precipitation is estimated by analyzing the echoes of these pulses. This analysis is carried out, for example, on the basis of a measurement of the reflectivity level over the resolution volume of the radar.
There are components that cannot be detected using reflectivity measurements. These components are mainly cloud droplets, atmospheric gases and the effects due to the non-uniformity of filling of the radar resolution volume.
Liquid water clouds consist of microscopic spherical water droplets that are encountered at positive but also negative temperatures. In the latter case, they are in a meta-stable supercooled state observable down to −42° C. The droplets play an essential role in the physics of precipitating systems as they are involved in the formation and growth of precipitation, for example hail. The water droplets have a low reflectivity, of less than −15 dBZ as indicated in the article by H. Sauvageot and J. Omar entitled Radar reflectivity of cumulus clouds, Journal of Atmospheric and Oceanic Technology, vol. 4, pages 264-272, 1987. They are therefore undetectable by the radar because of reflectivity below the detectable signal, although they do significantly attenuate microwaves, in particular in the X band and at higher frequencies, as explained in the article by O. Pujol, J.-F. Georgis, L. Féral and H. Sauvageot entitled “Degradation of radar reflectivity by cloud attenuation at microwave frequency”, Journal of Atmospheric and Oceanic Technology, vol. 24, pages 640-657, 2006.
Atmospheric gases are also undetectable by a radar and contribute to the attenuation of electromagnetic waves and therefore attenuation of the radar signal. In the frequency range in question, the attenuating gases are mainly dioxygen O2 and water vapor H2O, as indicated in the work by H. Sauvageot entitled “Radar Meteorology”, Artech House Publisher, 1992. Dioxygen has two absorption bands centered around 60 and 119 GHz. As for water vapor, this has two absorption bands with central frequencies approximately located at 22 and 183 GHz. It follows that these gases affect the propagation of microwaves, especially those in the X band.
The homogeneity of the target in the resolution volume is an implicitly accepted assumption in radar observations and in the algorithms conventionally used for correcting attenuation.
The measured reflectivity Zm, expressed in mm6·m−3, of a resolution volume Vr centered on a point M may be expressed by the following equation:
                                          Z            m                    ⁡                      (            r            )                          =                              Z            ⁡                          (              r              )                                ⁢          exp          ⁢                      {                          ln              ⁢                                                          ⁢              10              ×                              [                                                      -                    0                                    ,                                      2                    ⁢                                                                  ∫                        0                        r                                            ⁢                                                                        k                          ⁡                                                      (                            s                            )                                                                          ⁢                                                  ⅆ                          s                                                                                                                    ]                                      }                                              (        1        )            in which:    r is the distance expressed in km between the point M and the radar;    k denotes the specific attenuation expressed in dB·km−1; and    Z(r) is the unattenuated reflectivity at the distance r, also called the true reflectivity and expressed in mm6·m−3, which term contains useful information for the detection.
The specific attenuation k has three contributions: that of precipitations, that of cloud droplets and that of atmospheric gases. The first contribution can be calculated directly since precipitations are detectable, and does not form the subject matter of the first part of this patent. This first contribution will therefore be omitted in the rest of the discussion relating to the non-detectable components. However, it is of course taken into consideration later (in the second part of the patent).
In equation (1) the specific attenuation k depends on the distance r between M and the radar. No information about the geometry of the resolution volume Vr is involved in this equation. To be precise, k is an average specific attenuation in Vr. It follows that the measured reflectivity Zm is also a quantity which is smooth over the resolution volume in question. This smoothing effect necessarily has an influence on the attenuation correction and introduces a bias in the measurements. This situation is not taken into account in a simple manner in the solutions of the prior art. It is very difficult to correct it as what is measured remains a value smooth over a resolution volume. On account of the 3 dB aperture of the beam of an onboard radar, this aperture being for example equal to 4°, the resolution volume may be very large. Thus, at some 150 km away, its vertical extension amounts to about 10 km, so that, at average latitudes, the resolution volume contains the entire troposphere. The non-uniform filling of the resolution volume is then the general case. It may therefore be seen that this non-uniformity becomes a problem for observations at moderate and large distances.
Exploitation of the radar signal, in particular for estimating, remotely, and realistically, the hazardousness of a precipitating system, requires the attenuation due to the aforementioned causes to be optimally corrected so as to obtain a value of the reflectivity as close as possible to the actual reflectivity.
The attenuation by cloud droplets is considerable if the electromagnetic wave propagates within a precipitating system. This is for example the case for observations of convective cells embedded in an extended stratiform background. Likewise, observation of two convective cells aligned along the radial radar direction necessitates, in order to assess the hazardousness of the situation, correcting the attenuation by the undetectable component, represented by the cloud droplets. Thus, a hazard associated with an observed precipitating system may be underestimated, in particular when the radar operates in the X band and at higher frequencies.
Correcting the attenuation due to cloud droplets would be easy if they could be detected directly or even indirectly. Several indirect methods have been developed in this regard. A first method consists in using a dual-frequency radar, that is to say in making two observations located at the same point in the time space at different frequencies (Gosset and Sauvageot 1992). For example, the 10 GHz/35 GHz frequency pair is suitable for this. However, such technique is not free of ambiguity in the measurements, such as those associated with the confusion between non-Rayleigh effects and the attenuation. Furthermore, it is difficult to implement this technique for airborne radars that operate at a single frequency, especially for space requirement reasons.
An alternative approach enabling the non-Rayleigh ambiguity to be eliminated is based on the use of three frequencies, as explained in the article by N. Gaussiat, H. Sauvageot and A. J. Illingworth entitled “Cloud liquid water and ice content retrieval by multi-wavelength radar”, Journal of Atmospheric and Oceanic Technology, Vol. 20, pages 1264-1275, 2003. This mechanism is technically sophisticated, and even less realistic as regards airborne radars.
Thus, at the present time, in the context of radars operating at a single frequency, such as for example airborne radars, there is no exploitable method for correcting the attenuation of microwaves due to cloud droplets.
The attenuation by atmospheric gases can be neglected only for short-range observations, such not being the case, for example, in the context of civil aviation as a pilot must ascertain the meteorological hazard at distances in excess of 100 km.
The non-uniformity of the resolution volumes may itself be neglected for short-range observations and small apertures of the radar beams. Here too, this is not the case in civil aviation: the radars used have relatively large 3 dB apertures, for example 4°, and the information sought by the pilot is at least a few 100 km away from the aircraft.